Thanks for all the tips!

300 Woods Dr Fontana Dam NC 28733, the Fontana Village Resort, will hold boxes for thru hikers (confirmed from the Resort receptionist by phone today). They are open 24 hours and you don't need to stay there.

The AWOL guide suggests the Fontana Lodge- PO Box 68, Hwy 28 North, Fontana Dam NC 28733. The Hike Inn will hold drop boxes for guests, and their address is 3204 Fontana Road, Fontana Dam.